Systems for producing hydrocarbons from wellbores typically employ a well head, which includes a well head housing, connected atop surface casing extending into the earth from the top of the wellbore and cemented into place within the wellbore. During drilling and completion operations, a blowout preventer may be included atop the well head housing.
Generally, as a wellbore is drilled, successively smaller diameter casing strings are concentrically installed in the well bore at deeper depths, suspended from casing hangers landed, seated, and locked within the well head housing. The casing strings isolate the wellbore from the surrounding formation. The area between any two adjacent casings defines a casing annulus. Similarly, production tubing is typically concentrically installed within the inner casing, suspended from a tubing hanger landed and seated within the well head housing. The production tubing provides a conduit for producing the hydrocarbons entrained within the formation. An inner casing annulus is defined between the inner casing and the production tubing. Moving outward from the production tubing to the outermost casing, these various annuli are conventionally identified alphabetically as the A-annulus, B-annulus, C-annulus, etc.
Typically, each casing hanger is sealed within the well head housing by a mechanical seal assembly. Accordingly, the upper end of each casing is sealed from the adjacent casing. Likewise, cement is typically deposited about the lower end of each casing string to form a casing shoe, thereby sealing the annulus at the lower end of a casing string, with the result being that any fluid located within a casing annulus may become trapped. If fluid constrained within an annulus becomes pressurized, such as from a leak or thermal expansion, a pressure differential may overstress and/or rupture a casing or tubing wall. The phenomenon of trapped annulus pressure or annular pressure buildup is traditionally addressed by overdesigning casing strings and production tubing, with a concomitant cost penalty.